1. Field of the Invention
The present invention relates to an integrated multisound signal demodulator circuit.
A method of multisound transmission is currently in use in the television broadcasting systems in both Japan and U.S.A., and application of such technique is gradually spreading in other countries as well. This method enables reception of a stereophonic or bilingual broadcast, which is different from the conventional monaural audio transmission, due to multiplexing the signals. According to the multisound television broadcasting system adopted in Japan, two signals are multiplexed, and a stereophonic or bilingual broadcast is selectable by switching. Meanwhile, according to the multisound television (SAP) broadcasting system in U.S.A., three signals are multiplexed so that simultaneous stereophonic and bilingual broadcasts are possible. The background is based on the fact that the bilingual broadcasting has an extremely great importance since U.S.A. is a multilingual country. Although some differences are existent in the multiplexing techniques relative to the multisound broadcasting systems employed in countries other than Japan and U.S.A., the basic idea is exactly the same with regard to the purpose of enabling reception of both stereophonic and bilingual signals.
The apparati to be equipped with the function of multisound signal demodulation are principally television receivers and video tape recorders, which are gradually spreading into wider use today, and it is estimated that the function will shortly be included in all such apparatus except those of the standard class.
Now a description will be given on the output signal control system for a multisound signal demodulator in the television receiver. The demodulator used in such apparatus is formed into an IC (semiconductor integrated circuit). Presently, several kinds of integrated circuits are available for such demodulator, and employment thereof is advantageous in mountability and economy. Meanwhile, from the viewpoint of the designer the integrated circuit, it is desired that the multisound signal demodulator be usable for both a television receiver and a video tape recorder in common. However, in the current technical stage, the multisound signal IC-demodulator fails to satisfy completely the requirements for the video tape recorder, although capable of meeting most of the requirements for the television receiver.
Hereinafter this problem will be described in detail. The simplest multisound signal demodulator is equipped with a single-line output (two channels for stereo), for either of the stereo or bilingual broadcasts to be output, on the basis of the user's selection and the broadcasting mode. In this case, no particular difference exists between the requirements for the television receiver and the video tape recorder.
However, any higher-class apparatus is equipped with another output. FIG. 5 shows the constitution of such conventional multisound signal IC-demodulator with regard to its mode control. In FIG. 5. a multisound signal is fed to an external input terminal 2 of the multisound signal IC-demodulator 1, and a first or second sound signal is obtained from first and second external output terminals 3 and 4. Meanwhile, another output of the first and second external output terminals 3 and 4 is manifested at a third external output terminal 5. First and third external control terminals 6 and 7, for controlling the mode of the multisound signal IC-demodulator 1, are connected respectively to signal output terminals T1 and T3 of a switching signal generator 9. Control signals for the two external control terminals 6 and 7 may be produced by a variety of means. With regard to a typical example, where the first external control terminal 6 is used for controlling the selective changeover of the first and second sound signals, and the third external control terminal 7 is used for compulsorily rendering monaural the first sound signals of left and right channels output to the first and second external output terminals 3 and 4, an explanation will be given of the operation modes of the stereo/bilingual switching system as they have been adopted in Japan. For reception of a stereo broadcast, there are two modes relative to the output state, corresponding to the operation mode of the two external control terminals 6 and 7: i.e., one mode where first sound signals of left and right channels are output to the first and second external output terminals 3 and 4; and another mode where a monaural first sound signal is output to the first and second external output terminals 3 and 4, in response to a predetermined control signal supplied to the third external control terminal 7 in a particular case.
Meanwhile, for receiving a bilingual broadcast, there are two modes relative to the output state, corresponding to the operation mode of the two external control terminals 6 and 7, i.e., one mode where the stereo first sound signal is output to the first and second external output terminals and 4 while the second sound signal is output to the third output terminal 5; and another mode where a monaural second sound signal is output to the first and second external output terminals 3 and 4 while the monaural first sound signal is output to the third external output terminal 5.
Furthermore, in the simultaneous stereo/bilingual broadcasting system adopted in U.S.A., exemplary output modes are listed in Table 1 below with the contents of the output sound signals obtained from the first through third external output terminals 3-5 being determined in accordance with "L" (logic low level) and "H" (logic high level) of control signals supplied to the external control terminals 6 and 7 during a stereo/bilingual broadcast mode.
TABLE 1 __________________________________________________________________________ First and second output terminals Broadcast Terminal Terminal 3 and 4 Third output mode 6 7 L 3 R 4 terminal 5 __________________________________________________________________________ First sound H L Second sound L R (monaural) Stereo + First sound bilingual H H (compulsory) Second sound broadcast monaural) (monaural) Second sound First sound L L (monaural) (monaural) __________________________________________________________________________
The television receiver mentioned above is so devised as to enable, through an earphone or headphone terminal, the monitoring of another signal different from the signal to which the user is presently listening (the second sound signal in the case of listening to the first sound signal, or the first sound signal in the case of listening to the second sound signal).
As for the video tape recorder, other modes are required which are different from those in the multisound signal IC-demodulator used in the above-described television receiver. In a 1/2-inch video tape recorder where a high-fidelity audio characteristic is required, the technical trend is changing from the conventional baseband audio track recording toward a hi-fi method of multiplexing video signals by frequency modulation (hereinafter such recording portion will be referred to as the hi-fi channel). And the 8-mm video tape recorder, succeeding the 1/2-inch type, has one frequency-modulated multiplex channel (AFM channel) and two PCM channels for stereo.
In the video tape recorder having three audio channels as mentioned, selection of such audio channels for recording signals therein is determined by the user's intention and purpose. That is, in case there is need for a foreign language (principally English in Japan or Spanish in the U.S.A.), a first main sound signal is recorded in the hi-fi channel of the 1/2-inch VTR or in the PCM channel of the 8-mm VTR, while a second sound signal is recorded in the baseband audio track of the 1/2 -inch VTR or in the AFM channel of the 8-mm VTR. Meanwhile, if there is no need for recording the sound signal of a foreign language, the first sound signal is recorded on all of the entire channels, thus permitting reproducibility of the signals by a standard-class VTR (1/2-inch type without a hi-fi channel or by a 8-mm type without a PCM channel). Alternatively, a user, whose mother tongue is a foreign language, may record the second sound signal on all of the channels. Thus, the video tape recorders manufactured today are mainly of the type having two-line three audio channels. And it is ideal that selection of the audio channels for recording the first an second sound signals in the respective lines is adapted to comply with any combination, at the user's option.
However, the conventional multisound signal IC-demodulator outputs merely the signals of one line or is equipped with a third external output terminal to simultaneously provide at most another signal different from the output signals provided at the first and second external output terminals. Therefore, although no problem is existent with regard to the function for the television receiver the conventional demodulator is never adapted to meet the aforementioned requirements in a video tape reorder, and there has heretofore been no solution of the problem unless a functional compromise is made or in the case of the video tape recorder, a complicated external switching circuit is added.